Re: [PATCH v3 2/2] ASoC: Intel: Add period size constraint on strago board

[Takashi Iwai <tiwai@suse.de>]

On Wed, 12 Aug 2020 09:43:22 +0200,
Yu-Hsuan Hsu wrote:
> 
> Takashi Iwai <tiwai@suse.de> 於 2020年8月12日 週三 下午2:58寫道：
> >
> > On Wed, 12 Aug 2020 08:53:42 +0200,
> > Yu-Hsuan Hsu wrote:
> > >
> > > Takashi Iwai <tiwai@suse.de> 於 2020年8月12日 週三 下午2:14寫道：
> > > >
> > > > On Wed, 12 Aug 2020 05:09:58 +0200,
> > > > Yu-Hsuan Hsu wrote:
> > > > >
> > > > > Mark Brown <broonie@kernel.org> 於 2020年8月12日 週三 上午1:22寫道：
> > > > > >
> > > > > > On Tue, Aug 11, 2020 at 11:54:38AM -0500, Pierre-Louis Bossart wrote:
> > > > > >
> > > > > > > > constraint logic needs to know about this DSP limitation - it seems like
> > > > > > > > none of this is going to change without something new going into the
> > > > > > > > mix?  We at least need a new question to ask about the DSP firmware I
> > > > > > > > think.
> > > > > >
> > > > > > > I just tested aplay -Dhw: on a Cyan Chromebook with the Ubuntu kernel 5.4,
> > > > > > > and I see no issues with the 240 sample period. Same with 432, 960, 9600,
> > > > > > > etc.
> > > > > >
> > > > > > > I also tried just for fun what happens with 256 samples, and I don't see any
> > > > > > > underflows thrown either, so I am wondering what exactly the problem is?
> > > > > > > Something's not adding up. I would definitively favor multiple of 1ms
> > > > > > > periods, since it's the only case that was productized, but there's got to
> > > > > > > me something a side effect of how CRAS programs the hw_params.
> > > > > >
> > > > > > Is it something that goes wrong with longer playbacks possibly (eg,
> > > > > > someone watching a feature film or something)?
> > > > >
> > > > > Thanks for testing!
> > > > >
> > > > > After doing some experiments, I think I can identify the problem more precisely.
> > > > > 1. aplay can not reproduce this issue because it writes samples
> > > > > immediately when there are some space in the buffer. However, you can
> > > > > add --test-position to see how the delay grows with period size 256.
> > > > > > aplay -Dhw:1,0 --period-size=256 --buffer-size=480 /dev/zero -d 1 -f dat --test-position
> > > > > Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000
> > > > > Hz, Stereo
> > > > > Suspicious buffer position (1 total): avail = 0, delay = 2064, buffer = 512
> > > > > Suspicious buffer position (2 total): avail = 0, delay = 2064, buffer = 512
> > > > > Suspicious buffer position (3 total): avail = 0, delay = 2096, buffer = 512
> > > > > ...
> > > >
> > > > Isn't this about the alignment of the buffer size against the period
> > > > size, not the period size itself?  i.e. in the example above, the
> > > > buffer size isn't a multiple of period size, and DSP can't handle if
> > > > the position overlaps the buffer size in a half way.
> > > >
> > > > If that's the problem (and it's an oft-seen restriction), the right
> > > > constraint is
> > > >   snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
> > > >
> > > >
> > > > Takashi
> > > Oh sorry for my typo. The issue happens no matter what buffer size is
> > > set. Actually, even if I want to set 480, it will change to 512
> > > automatically.
> > > Suspicious buffer position (1 total): avail = 0, delay = 2064, buffer
> > > = 512 <-this one is the buffer size
> >
> > OK, then it means that the buffer size alignment is already in place.
> >
> > And this large delay won't happen if you use period size 240?
> >
> >
> > Takashi
> Yes! If I set the period size to 240, it will not print "Suspicious
> buffer position ..."

So it sounds like DSP handles the delay report incorrectly.
Then it comes to another question: the driver supports both SOF and
SST.  Is there the behavior difference between both DSPs wrt this
delay issue?


Takashi

> 
> Yu-Hsuan
> 
> >
> > > > > 2. Since many samples are moved to DSP(delay), the measured rate of
> > > > > the ring-buffer is high. (I measured it by alsa_conformance_test,
> > > > > which only test the sampling rate in the ring buffer of kernel not
> > > > > DSP)
> > > > >
> > > > > 3. Since CRAS writes samples with a fixed frequency, this behavior
> > > > > will take all samples from the ring buffer, which is seen as underrun
> > > > > by CRAS. (It seems that it is not a real underrun because that avail
> > > > > does not larger than buffer size. Maybe CRAS should also take dalay
> > > > > into account.)
> > > > >
> > > > > 4. In spite of it is not a real underrun, the large delay is still a
> > > > > big problem. Can we apply the constraint to fix it? Or any better
> > > > > idea?
> > > > >
> > > > > Thanks,
> > > > > Yu-Hsuan
> > > > >
> > >
>